Among other things, this disclosure provides an olefin oligomerization system and process, the system comprising: a) a transition metal compound; b) a pyrrole compound having a hydrogen atom on at the 5- position or the 2- and 5- position of a pyrrole compound and having a bulky substituent located on each carbon atom adjacent to the carbon atom bearing a hydrogen atom at the 5- position or the 2- and 5- position of a pyrrole compound. These catalyst system have significantly improved productivities, selectivities to 1-hexene, and provides higher purity 1-hexene within the C6 fraction than catalyst systems using 2,4-dimethyl pyrrole.